Indoleacetic acid (IAA) is a typical plant hormone. Its actions that have been demonstrated include, among others, promoting plant rooting, inducing stem and leaf elongation, inducing parthenocarpy and preventing aging. IAA is therefore a substance of considerable significance from the plant physiology viewpoint. It has been established that IAA is produced not only by plants but also by microorganisms and animals. As regards microorganisms, in particular, various investigations are in progress concerning the meaning of the production of plant hormones by bacteria occurring on or in the plant stem, leaf and root regions. Thus, for instance, it has been shown that Pseudomonas syringae pv. savastanoi infects olives and produces IAA, leading to tumor formation (Physiol. Plant Pathol., 13, 203-214, 1978). Furthermore, Agrobacterium tumefaciens infects dicotyledons and the T-DNA region of the Ti plasmid carried by this microorganism is integrated into the nucleus of said plants causing formation of tumors called crown galls. It has been revealed that the T-DNA contains a gene involved in the biosynthesis of IAA (Eur. J. Biochem., 138, 387-391, 1984).
It has been established that IAA is generally produced from tryptophan as a precursor via the following three biosynthetic pathways (Biol. Rev. , 48, 510-515, 1973).
1) Tryptophan.fwdarw.indoleacetamide.fwdarw.IAA; PA0 2) Tryptophan.fwdarw.tryptamine.fwdarw.indoleacetaldehyde.fwdarw.IAA; PA0 3) Tryptophan.fwdarw.indolepyruvic acid.fwdarw.indoleacetaldehyde.fwdarw.IAA.
The Pseudomonas and Agrobacterium species mentioned above have been shown to produce IAA by the biosynthetic pathway 1), and the enzymes involved therein and the genes coding therefor have been isolated (Proc. Natl. Acid. Sci. USA, 81, 1728-1732, 1984; ibid., 82, 6522-6526, 1985).
On the other hand, as regards the biosynthetic pathway 3), which is considered to be the main one in plants, neither the enzymes involved therein nor the genes coding therefor have been isolated because of instability of the intermediates involved. Accordingly, the mechanisms of control thereof have not been made clear as yet. As the above facts suggest, the plant physiology of IAA, a representative plant hormone, remains unknown in many respects and its functions have been known only insufficiently.